The book does a good job of explaining why controlling nodding is key:

Can you imagine James Bond fidgeting? How about tugging at his clothing, bobbing his head, or twitching his shoulders? How about hemming and hawing before he speaks? Of course not. Bond is the quintessential cool, calm, and collected character. He epitomizes confidence.

This kind of high-status, high-confidence body language is characterized by how few movements are made. Composed people exhibit a level of stillness, which is sometimes described as poise. They avoid extraneous, superfluous gestures such as fidgeting with their clothes, their hair, or their faces, incessantly nodding their heads, or saying “um” before sentences.